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M2 Impulse

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I'm stuck on the bottom bit of question 11- where you need to find the speed after the second collision where the KE is 4/5mu2?
I've included my attempt but it is in terms of q (the speed of ball B) which is incorrect?

Thanks :wink:
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Reply 1
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Zacken
22
Original post by AvM4N
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I'm stuck on the bottom bit of question 11- where you need to find the speed after the second collision where the KE is 4/5mu2?
I've included my attempt but it is in terms of q (the speed of ball B) which is incorrect?

Thanks :wink:


Use conservation of linear momentum to get another equation in p and q.
Reply 2
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AvM4N
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Original post by Zacken
Use conservation of linear momentum to get another equation in p and q.


So 4MU - 2MU= -4Mp+2Mq ???
Reply 3
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Zacken
22
Original post by AvM4N
So 4MU - 2MU= -4Mp+2Mq ???


Something like that, yeah.
Reply 4
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AvM4N
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Original post by Zacken
Something like that, yeah.


But when I do that i get-
q= u - 2p
so p= √10/5u - 1/2(u - 2p)= √10/5u - 1/2u + p, so the P cancels out
what have i done wrong??
Reply 5
Avatar for Zacken
Zacken
22
Original post by AvM4N
But when I do that i get-
q= u - 2p
so p= √10/5u - 1/2(u - 2p)= √10/5u - 1/2u + p, so the P cancels out
what have i done wrong??


Oh dear.

p2=25u2−14q2⇏p=2u25−q24p^2 = \frac{2}{5}u^2 - \frac{1}{4}q^2 \not\Rightarrow p = \sqrt{\frac{2u^2}{5}} - \sqrt{\frac{q^2}{4}}

Do you think that 22=2+2⇒2=2+22^2 = 2 + 2 \Rightarrow 2 = \sqrt{2} + \sqrt{2}?
Reply 6
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AvM4N
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5
Original post by Zacken
Oh dear.

p2=25u2−14q2⇏p=2u25−q24p^2 = \frac{2}{5}u^2 - \frac{1}{4}q^2 \not\Rightarrow p = \sqrt{\frac{2u^2}{5}} - \sqrt{\frac{q^2}{4}}

Do you think that 22=2+2⇒2=2+22^2 = 2 + 2 \Rightarrow 2 = \sqrt{2} + \sqrt{2}?


Where have i done that? I thought in order to square root the RHS I had to square root everything??
Reply 7
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Zacken
22
Original post by AvM4N
Where have i done that? I thought in order to square root the RHS I had to square root everything??


Yes, you square root everything. But the square root everything does not mean you square root each individual term individually.

It's in your second to last line moving into your last line.

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